The present invention relates to a method and apparatus for determining the characteristics of reservoir rocks and particularly, the characteristics of core samples that are removed from reservoir rocks.
One method that has been used for determining the characteristics of reservoir rocks is the measurement of capillary pressure. In particular, the mercury capillary pressure response of the reservoir rock has been measured from which estimates have been made of the permeability of the rock as well as the pore volume. One mercury capillary pressure method and apparatus is described in U.S. Pat. No. 2,604,779. In this method the rock sample or core is placed in a closed vessel, the vessel is evacuated and then partially filled with mercury to cover the core. Mercury is then forced into the vessel at a constant pressure while plotting the volume of mercury versus the pressure in the vessel. This provides a plot of the mercury capillary pressure versus volume of the core from which various core characteristics may be determined. For example, total pore volume and an indication of the permeability of the core can be determined.
In an article appearing in the Journal of Geophysical Research, Volume 71, No. 12, June 15, 1966, at pp. 2911, entitled "Flow of Fluids Through Porous Mediums", there is described experiments with air/water interfaces moving through simple synthesized permeable specimens. The authors observed that the interface did not move continuously through the synthesized sample but rather in a non-continuous manner from pore to pore. The abrupt movements of the interface resulted in pressure drops in the pressure measured in the sample holder.
While the authors observed the non-continuous manner in which the interface moved through the synthetic permeable specimen, they did not utilize actual rock samples. Furthermore, they relied upon air/water interfaces. For the results to be useful the interface must be conventional water/oil interface that occurs in reservoir rocks or mercury/air interface. Further, the pores that occur in reservoir rocks are smaller than those present in the authors' synthetic sample by a factor of at least 10 to 100. The equipment and procedures used by the authors would be incapable of measuring the pressure and volume changes occurring in an actual rock sample.